Answer an empty set-element result as NULL rather than raising

[estebanzimanyi]

MEOS returns the empty set as a NULL pointer, so setMinus('{3}'::intset, 3),
setIntersection('{1}'::intset, 2) and their bigint, float, date, timestamptz
and text twins raise "Invalid Input Error: Null pointer not allowed" where the
answer is the empty set.

The generated body for a set with a scalar element marshals its result through
SetToBlobN, which carries a NULL pointer to SQL NULL. That is what PostgreSQL
answers: Setop_set_base in mobilitydb/src/temporal/set_ops.c, the shared
wrapper over intersection_set_value and minus_set_value, reads
"if (! result) PG_RETURN_NULL()". The (Set,Set) body beside it already answers
NULL for the same emptiness.

The registered surface is unchanged: duckdb_functions() reports the same 11625
rows, the emitted registration set is identical at 6065 lines, and the line
movement in the generated chunks is their split points shifting. setMinus of
{1,3,5} and 3 answers {1, 5}, setUnion of {1,3} and 9 answers {1, 3, 9}, and
setIntersection of {1,3} and 3 answers {3}.